Hypovolemic Hypoosmolar Hyponatremia

Aka: Hypovolemic Hypoosmolar Hyponatremia

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II. Pathophysiology

  1. Total Body Sodium Deficit exceeds water losses
  2. Decreased Extracellular fluid volume
    1. Increased proximal tubule fluid reabsorption
    2. Decreased distal segment flow where dilution occurs
  3. Hypovolemia stimulates non-osmotic fluid conservation
    1. Thirst
    2. ADH secretion

III. Differential Diagnosis

  1. Often difficult to distinguish Iso- from Hypovolemic
  2. See Isovolemic Hypoosmolar Hyponatremia

IV. Labs

  1. Urine Sodium < 20 meq/L
    1. Non-Renal Sodium Loss (e.g. Vomiting, Diarrhea, severe burns)
    2. Other lab findings
      1. Urine Osmolality >400 mOsm/kg
      2. Fractional Excretion of Urea <35%
        1. Use instead of Urine Sodium in patients on Diuretics
        2. Carvounis (2002) Kidney Int 62(6): 2223-9 [PubMed]
  2. Urine Sodium > 20 meq/L
    1. Renal Sodium Loss (e.g. Diuretics, RTA, Adrenal Insufficiency)
    2. Other lab findings
      1. Urine Osmolality <400 mOsm/kg

V. Causes: Non-Renal Losses (Sodium appropriately conserved)

  1. Gastrointestinal losses
    1. Vomiting
    2. Diarrhea
  2. Third space losses
    1. Pancreatitis
    2. Pleural Effusion
  3. Skin Losses
    1. Severe burns

VI. Causes: Renal Losses (Renal inappropriate Sodium losses)

  1. Diuretics
    1. Thiazide Diuretics
    2. Loop Diuretics
  2. Renal Tubular Acidosis
    1. Hyperchloremic Metabolic Acidosis
    2. Increased urinary pH
    3. Fractional Excretion of Bicarbonate >15-20%
  3. Salt-losing Glomerulonephritis
    1. Chronic Renal Insufficiency on Low Sodium Diet
    2. Severe interstitial Kidney disease
      1. Polycystic Kidney Disease
      2. Medullary cystic disease
      3. Chronic Pyelonephritis
  4. Mineralocorticoid and Glucocorticoid deficiency
    1. Adrenal Insufficiency (Addison's Disease)
  5. Osmotic Diuresis (Bicarbonate, Glucose, Ketones)
    1. Excess osmotically active solutes in urine
    2. Draws increased Sodium and water renal losses
  6. Cerebral salt wasting (head injuries, Intracranial Hemorrhage)
    1. Diagnosis of exclusion

VII. Management

  1. See Hyponatremia Management
  2. Stop all Diuretics
  3. Correct non-renal fluid losses
  4. Replace Sodium deficit
    1. Calculate Total Body Sodium Deficit
    2. Use Normal Saline (0.9% = 150 meq/L)
    3. Replace one third Sodium deficit over first 6-8 hours
    4. Replace remaining Sodium deficit in next 24-48 hours

VIII. References

  1. Kone in Tisher (1993) Nephrology, p. 87-100
  2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  3. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  4. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
  5. Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]

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